Chromosomal and chromatid

The disadvantages of cytogenetic assays are that they are time-consuming and that negative results indicate only that further studies must be conducted with S-9 activation and with multiple fixations to account for possible different sensitivities of stages of the cell cycle. Furthermore, the scoring of 300 cells per point for chromosomal and chromatid aberrations is very difficult, requires much expertise, and is necessary to establish an accurate result. 

The absence of PARP-2 in PARP-2 Mouse embryonic fibroblasts (MEFs) cells has no effect on mean telomere length measured by Q-FISH. Telomerase activity is also unpermrbed in these cells compared to wild type cells. However, there is a s nificant spontaneous increase in the signal-free ends in PARP-2 cells (Fig. 9B), as well as in the heten eneity of telomere lengths per chromosome, in addition to an increase in the fi equency of spontaneous chromosomes and chromatid breaks. These observations are suggestive of a telomere dysfunction in the absence of PARP-2. 

TABLE 25.2 Chromosomal and Chromatid Types ofRearrangements in Control Experiment... 

Mutagenic - in vitro (cell culture, microsomes), dose variable. Frequent breaks at chromosome and chromatid 263... 

J. Rommelaere, M. Susskind, and M. Errera, Chromosome and chromatid exchanges in Chinese hamster cells, Chromosoma 41, 243-257 (1973). 

The chromatid separation process has also remained mysterious. It is an autonomous process that does not direcdy depend on the mitotic spindle (Wilson 1925, Mazia 1961). This is most vividly seen in cells whose spindles have been destroyed by spindle poisons such as colchicine. In many organisms, in particular in plant cells, the cell cycle delay induced by colchicine is only transient and chromatids eventually split apart in the complete absence of a mitotic spindle (Mole-Bajer 1958, Rieder Palazzo 1992) (Fig. 2). Mitosis in the presence of colchicine or colcemid (known as c-mitosis) leads to the production of daughter cells with twice the normal complement of chromosomes. This process is routinely used for manipulating plant genomes and may contribute to the therapeutic effects of taxol in treating breast cancer. 

The nucleus of all eucariotic cells contains the carrier of the genetic information in the chromosomes. It is possible to visualize the chromosomes and analyze their number and pattern during a special period of cell division (the metaphase). Alterations from their normal shapes are observed as structural chromosome aberrations. These are chromosome type aberrations (terminal and interstitial deletions, dicentrics and rings), chromatid aberrations (gaps, breaks and exchanges) and sister chromatid exchanges. Spontanous frequencies of such chromosome... 

The classification and nomenclature of the International System for Human Cytogenetic Nomenclature (ISCN, 1985) as applied to acquired chromosome aberrations is recommended. Score sheets giving the slide code, microscope scorer s name, date, cell number, number of chromosomes and aberration types should be used. These should include chromatid and chromosome gaps, deletions, exchanges and others. A space for the vernier reading for comments and a diagram of the aberration should be available. 

During mitosis, aU the DNA is highly condensed to allow separation of the sister chromatids. This is the only time in the ceE cycle when the chromosome structure is visible. Chromosome abnormalities may be assessed on mitotic chromosomes by karyotype analysis (metaphase chromosomes) and by banding techniques (prophase or prometaphase), which identify aneu-ploidy, translocations, deletions, inversions, and duplications. 

Cytogenetic studies have been conducted using bone marrow cells of rats following inhalation exposure to 1,4-dichlorobenzene (Anderson and Richardson 1976). Three series of exposures were carried out (1) one exposure at 299 or 682 ppm for 2 hours (2) exposures at 75 or 500 ppm, 5 hours per day for 5 days and (3) exposures to 75 or 500 ppm, 5 hours per day, 5 days per week for 3 months. Bone marrow cells from both femurs were examined for chromosome or chromatid gaps, chromatid breaks, fragments, or other complex abnormalities. In all three experiments, exposure to 1,4-dichlorobenzene failed to induce any effects indicative of chromosomal damage. Other genotoxicity studies are discussed in Section 2.5. 

Stmctural chromosome aberrations may be of two types, chromosome or chromatid. A chromo-some-type aberration is a stmctural chromosome damage expressed as breakage, or breakage and... 

Karelovaa J, Jablonickaa A, Gavora J, et al Chromosome and sister-chromatid exchange analysis in peripheral lymphocytes, and mutagenicity of urine in anesthesiology personnel. Int Arch Occup Environ Health 64(4) 303-306, 1992... 

Occupational exposure of 20 workers to pentachlorophenol at concentrations that ranged from 1.2 to ISOpg /m for 3-34 years did not result in any increased incidence of sister chromatid exchanges or chromosomal aberrations. In another report, significant increases in the incidence of dicentric chromosomes and acentric fragments were detected in the peripheral lymphocytes of exposed workers the frequency of sister chromatid exchanges was not increased." ... 

Aberrations in chromosomes or chromatids, which are sometimes microscopically visible, may arise during mitotic division when newly divided chromosomes fail to separate or do so incorrectly. The absence of a chromosome is usually lethal, and an excess is often poorly tolerated, giving rise to serious defects. Aberrations of the sex chromosomes are more readily tolerated, however. Chromosome aberrations may be caused by foreign compounds as indicated in the section on mutagenesis (see chap. 6). However, those cells with aberrations seem to be rapidly eliminated and so may contribute to cell death rather than a heritable mutation. 

Figure 6.45 Crossover at meiosis. Two homologous chromosomes are shown aligned in the top line. Each consists of two chromatids, joined together at the centromere. A and B represent genes on one chromosome, and a and b the corresponding alleles on the other chromosome. After recombination two new gene combinations are apparent. The middle and bottom lines represent other possible methods of recombination. Source From Refs. 12 and 13.

Chromosome aberrations have been reported in Syrian hamster embryo (SHE) cells after treatment with a DMSO extract of tryptophan pyrolysate. At a dose of 30 pg/ml, 69 exchanges and 29 chromosome or chromatid breaks were observed among 200 metaphases. Gaps and minutes were noted as well (33). Sasaki, et al. (34) also observed chromosome aberrations in human and Chinese hamster cell lines after exposure to Trp-P-1 and Trp-P-2. 

Chromosomal aberrations include both numerical and structural aberrations. Numerical aberrations are changes in the number of chromosomes of the normal number characteristic of the animals utilized (aneugenicity). Structural aberrations are classified into two types, chromosome or chromatid aberrations (clastogenicity). Chromosomal mutations and related events are the cause of many human genetic diseases and there is evidence that chromosomal mutations and related events are involved in cancer development. 

This in vitro cytogenetic test is a clastogenicity test system for the detection of chromosomal aberrations in cultured mammalian cells or primary cultures. Chromosomal aberrations may be either structural or numerical. However, because cytogenetic assays are usually designed to analyze cells at their first posttreatment mitosis and numerical aberrations require at least one cell division to be visualized, this type of aberration is generally not observed in a routine cytogenetic assay. The best estimate of aberration frequency is the first cell division after the start of treatment. Structural aberrations are of two types chromosome or chromatid aberrations. 

In a series of in vitro experiments on a human lymphocyte culture system, it was reported that chlorine induced chromatid and chromosome breaks, translocations, dicentric chromosomes, and gaps. 

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